This invention relates to a process for the continuous preparation of organic mono- or polyisocyanates by the reaction of mono- or polyamines corresponding to the mono- or polyisocyanates with a solution of phosgene in organic solvents at elevated temperatures.
It is known to prepare reaction mixtures of organic amine solutions and organic phosgene solutions in mixers with movable parts, such as rotary pump mixers. E.g., DE-AS 2,153,268 or U.S. Pat. No. 3,947,484. Due to the toxicity of phosgene, leakage at the points where the shafts pass through the apparatus can be particularly hazardous. Moreover, since the reaction also produces solids, the formation of deposits that cake to the surface of the apparatus may be unavoidable.
As a result, attempts have been made to find processes by which mixing can be carried out without moving parts. Thus, according to DE-OS 2,950,216, the two reactants impinge on each other in the form of fans of jet sprays in a cylindrical mixing chamber. Not only does this process require high inlet pressures, but blockages may also occur because of the dead zones of the mixing chamber where no flow takes place.
It is also known (e.g., U.S. Pat. No. 3,226,410) that the amine solution can be injected into a stream of phosgene solution in a pipe by means of apertures arranged laterally in the pipe. Since a low concentration of reactant is required for producing acceptable yields, the quantity of isocyanate produced is also small in proportion to the quantity of solvent. The large amount of energy consumption required for solvent recovery is an unsatisfactory aspect of the process. Moreover, build-up of layers of solids on the wall cannot always be avoided.
Since the known processes must be carried out with highly diluted reactants and since the frequent blockages force long periods of standstill for cleaning the apparatus, the known processes are uneconomical.
Thus, a new process for the continuous preparation of organic mono- or polyisocyanates in which the quantity of auxiliary solvent used could be considerably reduced while avoiding difficulties arising from the formation of solid deposits on the apparatus (and resultant blockages) would be desirable. Also desirable would be a new process that would dispense with the use of moving parts, thereby eliminating the above-mentioned hazards due to toxicity. The present invention provides a solution to these problems. In the process of this invention, the reaction mixture is prepared by bringing together the amine component, optionally as a solution, with the phosgene solution in a special nozzle of the type described hereinafter.